In the 1990s, the practical application of an MR (Magneto-Resistive effect) head and a GMR (Giant Magneto-Resistive effect) head triggers a dramatic increase in a recording density and a recording capacity of an HDD (Hard Disk Drive). However, since the beginning of the 2000s, a problem of thermal fluctuation of a magnetic recording medium is exposed, and therefore, a rate of increase in the recording density temporarily slows down. Still, recently, the HDD recording density has been increasing about 40% per annum, as the perpendicular magnetic recording more advantageous, in principle, for high-density recording than longitudinal magnetic recording was put into practical use in 2005.
However, the realization of such a high recording density is considered not easy even by using a perpendicular magnetic recording method, because the problem of thermal fluctuation is exposed again.
As a recording method to solve the above problem, a “high-frequency magnetic field assisted magnetic recording method” has been suggested. In the high-frequency assisted magnetic recording method, a high-frequency magnetic field at a frequency in the neighborhood of the resonant frequency of a magnetic recording medium, which is sufficiently higher than the recording signal frequency, is locally applied to the magnetic recording medium. As a result, the magnetic recording medium resonates, and the coercivity (Hc) of the magnetic recording medium subjected to the application of the high-frequency magnetic field is reduced to not more than half the original value. Thus, the high high-frequency magnetic field is overlapped with the recording magnetic field, whereby magnetic recording on a magnetic recording medium having a higher coercivity (Hc) and a higher magnetic anisotropy energy (Ku) is allowed.
However, it is necessary to apply a large current to a spin torque oscillator (STO) as a source to generate a high frequency for high-frequency magnetic field assisted recording. Accordingly, the generated Joule heat results in difficulty in obtaining long-term reliability. In order to increase, particularly high-frequency magnetic field intensity, a demand for a large current is increased as a magnetovolume (Mst) of an oscillation layer (FGL) is increased. Consequently, it is desired to reduce the oscillation drive voltage of the STO and achieve a strong high-frequency magnetic field and long-term reliability.